There has been used a power supply system configured such that a chargeable and dischargeable secondary battery can supply power to a load and the secondary battery can be charged even during an operation of the load when necessary. Typically, this kind of power supply system is mounted on a hybrid vehicle and an electric vehicle that employ an electric motor driven by the secondary battery as a drive power source.
In this power supply system, accumulated electric power of the secondary battery is used as drive electric power for an electric motor that is a drive power source. Also, the secondary battery is charged with electric power generated by regeneration of this electric motor, electric power generated by an electric generator that generates the power according to rotation of an engine, and the like. In this kind of power supply system, a state estimating device for a secondary battery is typically required to obtain correctly a charging rate (i.e., an SOC: State Of Charge) with respect to a full charge state. More specifically, it is necessary to restrict excessive charging and discharging of the secondary battery by successively and correctly estimating the charging rate of the secondary battery even during the charging/discharging and immediately after the charging/discharging. In addition, when the state of the secondary battery gradually changes (i.e., the secondary battery is deteriorated) with the use of the secondary battery, battery parameters (internal resistance, full charge capacity, and the like) of the secondary battery change. Therefore, it is required to accurately estimate the state of the secondary battery corresponding to such change over time.
For example, Japanese Patent Laying-Open No. 2005-37230 (Patent Document 1) discloses deterioration detection device and method for detecting deterioration of a battery by comparing a difference between a measured SOC obtained from a summed value of a measured battery current and an estimated SOC obtained from a summed value of an estimated battery current, with a predetermined amount.
For example, Japanese Patent Laying-Open No. 2003-224901 (Patent Document 2) discloses a method of managing a battery capacity, including the steps of calculating an SOC of a battery from an open-circuit voltage of the battery in a non-load state; calculating a summed discharge current amount of the battery in a load state until the battery reaches the non-load state; and calculating a total real capacity of the battery based on the calculated SOC and the calculated summed discharge current amount.
For example, Japanese Patent Laying-Open No. 2005-269760 (Patent Document 3) discloses a charging battery that can improve accuracy of calculating a remaining capacity. According to this document, the charging battery includes: summed discharge amount calculating means for calculating and storing a summed discharge amount when the charging battery is in a discharge state; element data storing means for storing data of circuit elements in an equivalent circuit of the charging battery; estimated discharge characteristics calculating means for obtaining estimated discharge characteristics in accordance with a charging rate from the data in the element data storing means and a present discharge current value; charging rate calculating means for calculating a present charging rate of the charging battery in accordance with a remaining capacity of the charging battery when discharging starts and the summed amount calculated thereafter by the summed discharge amount calculating means; and discharge characteristics correcting means for calculating a ratio of a difference between a voltage value of the charging battery at the present charging rate and a voltage value thereof in a full charge state, to a difference between a voltage value obtained from the estimated discharge characteristics corresponding to the present charging rate and the voltage value of the charging battery in the full charge state, and correcting the estimated discharge characteristics in accordance with the ratio. The charging battery obtains the remaining capacity of the charging battery when discharging starts as described above, based on the corrected estimated discharge characteristics.